Mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) cause Rett syndrome (RTT) and a host of other mental retardation syndromes in both males and females. RTT results from loss-of-function mutations of MECP2, which binds to target DNA and recruits Sin3A and histone deacetylases to silence transcription. MeCP2 is part of a larger family of genes, the methyl-CpG-binding domain (MBD) family, which is conserved from fly through humans. Overexpression of MECP2 in Drosophila using the UAS-GAL4 system produces a phenotype, possibly by competing with MBD genes. I propose to continue characterization of the full-length lines and create additional transgenic flies: two lines which overexpress MECP2 mutations commonly found in Rett patients and one line which produces MeCP2 without the MBD region. Secondly, I will characterize the Drosophila MBD gene CG10042 for native RNA and protein expression patterns. Then I will investigate its function through P-element mutagenesis and RNAi with genomic eDNA hybrids. I will characterize all resulting phenotypes and use these strains to identify modifiers of MeCP2 and/or CG10042 through an F 1 modifier screen to gain insight into pathways of MeCP2 function, which until now has been studied only in vitro. Through the proposed genetic studies, I hope to identify proteins that can be then studied to investigate the mechanisms of the mental and neurological dysfunction in Rett and related disorders.